1. The Field of the Invention
Exemplary embodiments of the invention relate to transportation. More particularly, exemplary embodiments relate to transportation by rail. More particularly still, exemplary embodiments of the invention relate to stacked railway ties.
2. The Relevant Technology
Railway systems play an important role in North American and other worldwide economies. Railway systems can be adapted to run over land or water and thus provide a quick, reliable, convenient, and generally inexpensive method of transportation for both products and people.
Railway systems run over sets of tracks, and each track is made up of one or more rails. A railway tie is a well-known apparatus for use in supporting a rail. In practice, multiple ties are spaced to support sections of the rail so as to form the track over which a train may run. Railway ties may be made of a variety of materials including timber, reinforced concrete, composites, such as plastic composites or carbon, or steel. A railway tie may comprises a solid block which has upper and lower surfaces for contacting a rail and ballast, respectively. In operation, the railway tie is either placed on the ballast or partially submerged within the ballast, and the rail is secured to the upper surface of the railway tie. For example, when the railway tie is comprised of timber or plastic composite, the rail is secured to the railway tie via a tie plate.
As a train moves along a rail and across the railway ties, the tie supports the weight of the train and helps to transfer at least a portion of the load to ballast. Train movement also creates frictional forces and vibrations which may cause the railway ties to shift position or which may cause ballast to migrate and thereby cause cavities or otherwise reduce the ballast in contact with the railway tie. If the ballast migration is left uncorrected, the railway tie may be unable to effectively transfer load to the ballast and may fail. Alternatively, the shift in the ballast may cause the railway tie and the rail to fall out of alignment. Failure or misalignment of a railway tie can cause misalignment of the track, which can ultimately result in train derailment. To reduce ballast migration, and ultimately to prevent derailment, the crib (the void between adjacent ties) can be filled with additional ballast. To the extent the additional ballast does not effectively prevent ballast migration, expensive processes may be necessary to refill cavities, or alternatively, removal and/or replacement of the ties may be required.
Commonly, equipment such as switch rods, electric wire conduits, or other track components are placed or run between ties. When this equipment is in place between the ties, the crib is reduced. With the crib reduction there is less ballast filled in between railway ties, and greater ballast migration may result under the ties. One approach to resolve this is to allow for greater clearance for the equipment by raising the railway tie, and thereby reducing or eliminating the partial submersion within the ballast. While reducing the submersion of the railway tie allows for greater clearance and more ballast to fill the crib, the contact with ballast below the tie is reduced, which increases the tendency for cavities to form. Reducing the submersion of the tie also reduces the railway tie's resistance to lateral and longitudinal movement or displacement.
Railway ties have a limited service life before replacement becomes necessary. When the rail is connected to the railway tie, the weight of the rail and passing trains stresses the tie, with the stress being most concentrated near the areas where the rail contacts the tie and where the tie contacts ballast. Additional stress is placed on the tie from the removal and/or replacement of rails. Over time, the continual and cyclical loads on the stressed railway tie may cause the tie to fail, so as to require replacement. Additionally, train derailment may uproot a railway tie or may sufficiently damage a tie so as to necessitate replacement. When the service life ends and replacement is necessary, the tie is completely removed and a new tie is placed in or on the ballast. Often, where ballast migration occurred, replacement may be time consuming and/or expensive because ballast may need to be refilled and repacked before the new tie can be positioned.
Railway ties made of timber and steel have been used for more than a hundred years, and railway ties made with reinforced concrete have been used for the last thirty years. Traditionally, steel ties were more expensive than concrete or timber ties, but had a longer service life. Due in part to converging prices of steel and timber, the popularity of steel ties is increasing even where a timber tie has been treated with creosote to improve its service life. In part, this increased popularity is the result of other cost savings that can be realized by using steel ties. For example, steel ties are lighter than the timber or concrete counterparts so transportation costs are reduced. Because of the reduced weight, the dead loads that must be supported by bridges and foundations for railway tracks are also reduced. Additionally, steel ties wear better than timber ties in humid, wet environments, are not affected by insect infestation, do not suffer from plate cutting or spike kill, require less ballast per mile, allow greater load spreading capabilities, can be spaced at greater distances, are recyclable, require less material handling, and have better derailment survivability than either timber or concrete ties.
Despite the advantages that steel offers, timber ties are the traditional choice for most railroad applications, and are often used where the track carries electrical circuits for electrical signal systems or to reduce vertical pullout. Approximately twenty percent of the track mileage in North America has track signal systems using track circuits. A track circuit uses the rails as the conductors and the train wheels and axles as the switch for activating signals. Unlike timber ties, steel ties can conduct electricity. Thus, timber ties are more commonly used on tracks employing track circuits because steel ties can complete the track circuit and disrupt the signaling systems. Where track signal systems are necessary in conjunction with conductive ties, such as steel ties, insulator systems are required to reduce or prevent short-circuit of the electrical current through the conductive tie material. All concrete ties require rail seat insulation systems to protect the concrete from damage that would be imparted by direct contact with the steel rail regardless of track signal requirements.